TyroTuner is a microphone-based utility that tunes the standard six-string guitar. Select what string you want to tune, and then play the guitar string...the needle will tell you whether you need to tune up or down.

The green zone represent +/- 5 centitones, within the just-noticeable-difference.

TyroTuner is a rather simple but very nicely presented chromatic tuner app for iPhone and iPad that offers an easy-to-interpret display so that you can tune your guitar quickly and easily regardless of your level of proficiency.